1. Field of the Invention
This invention relates to electronically scanned radar systems and to methods for near-continuous tracking of targets at the same time as the radar operates in a conventional surveillance mode. In particular, this invention relates to radar systems using a dispersive antenna with a hopped frequency transmitter.
2. Description of the Prior Art
Electronically scanned radar systems are employed in many applications when targets must be tracked using part of the resources (bandwidths, beams, frequencies, etc.) at the same time that the radar system uses its remaining resources to scan the space around it.
In a typical example, the radar system may be required to rotate continuously in azimuth thus providing complete azimuth coverage. Such known radar systems have not been capable of continuous track-while-scan, and have relied on elaborations in the elevation plane such as frequency-scanning or multi-lobed beam structures to provide accurate elevation location. These radar systems have accordingly been used principally in surveillance roles. In cases where close and rapidly approaching targets must be tracked, phased arrays with complex phase-shifting modules have been used. Because of the requirements for narrow pencil beams, the phased arrays have employed large aperture antennas whose radiating and receiving apertures are densely populated by phase-shifting modules. For reasons of scan and sidelobe optimization, these modules must be spaced at not much more than 0.5 wavelength apart. For a 1o.times.1o pencil beam, there would be required some 14,000 modules, resulting in considerable expense.
It has therefore been desirable for some considerable time to provide a beam steering system in the azimuth plane that can compensate for the conventional rotation of the antenna beam, thereby permitting nearcontinuous track-while-scan, and at the same time permitting an appropriate elaboration of the elevation beam-forming design to provide elevation information of targets.